The present invention relates to an electric power steering apparatus using a motor as the source of steering assist force.
FIG. 1 is a cross sectional view showing the structures of a conventional electric power steering apparatus.
As shown in FIG. 1, for example, an electric power steering apparatus for automobiles comprises a first steering shaft 101 connected to a steering wheel 100 for steering; a second steering shaft 103 whose upper end portion is connected coaxilally with the lower end portion of the first steering shaft 101 through a torsion bar 102 and whose lower end portion is connected to a steering mechanism joined to wheels; a torque sensor 104 for detecting a torque applied to the first steering shaft 101 by rotation of the steering wheel 100, based on twist of the torsion bar 102; a steering assist motor driven according to the result of detection given by at least the torque sensor 104; and a reduction mechanism which is joined to an output shaft of the motor and includes a worm 105 and a worm wheel 106 for decelerating rotation of the output shaft and transmitting the resultant rotation to the second steering shaft 103. Such an electric power steering apparatus for automobiles is constructed to reduce the driver""s labor for steering by assisting the operation of the steering mechanism according to the rotation of the steering wheel 100 with the rotation of the motor.
The worm 105 constituting the reduction mechanism is positioned to cross the axis of the second steering shaft 103 and supported in a housing 107 through a pair of roller bearings. The second steering shaft 103 to which the worm wheel 106 is attached is supported in the housing 107 through a pair of roller bearings 108.
The worm wheel 106 includes a circular tooth member which is made of a synthetic resin and has teeth meshing with the worm 105, and a metal fitting member to be fitted into the circular tooth member. The worm wheel 106 is made of a synthetic resin, so that noise caused by meshing with the worm 105 is reduced and the workability in making the teeth is improved.
An electric power steering apparatus using a motor as the source of steering assist force as mentioned above is employed mainly by midget cars and relatively small cars with an engine displacement of around 1300 cc, for example. On the other hand, relatively large cars with an engine displacement of around 2000 cc or more, for example, often employ a hydraulic steering apparatus using a hydraulic pump connected to the engine as the source of steering assist force.
By the way, since an engine as a travel driving source is also used as a hydraulic pump driving source of the hydraulic steering apparatus, even if steering assistance is not intended, the hydraulic pump is driven by the engine, and thus hydraulic steering apparatus does not meet a demand for low fuel consumption. Moreover, there is an increasing demand for applying en electric power steering apparatus to cars with an engine displacement of around 1300 cc to 2000 cc. According to this demand, it has been considered to apply the electric power steering apparatus to relatively large and heavy vehicles or tracks in the near future, and thus steering assistance with higher power compared to a conventional steering assist force will be required. Accordingly, in the case where at least a part of the worm and worm wheel constituting the reduction mechanism is formed by a synthetic resin, the high-heat-resistant, high-strength worm and worm wheel are necessary.
An object of the present invention is to provide an electric power steering apparatus capable of solving the above problem.
An electric power steering apparatus according to the first invention is an electric power steering apparatus for assisting steering by transmitting an output of a steering assist motor to a steering mechanism through a driving gear and a driven gear meshing with the driving gear, and characterized in that at least one of the driving gear and driven gear is formed by a thermosetting synthetic resin filled with aramid fibers.
According to the first invention, since one or both of the driving gear and driven gear are formed by a thermosetting synthetic resin with an excellent heat resistance, even when the reduction mechanism is used in an engine room or the temperature of the reduction mechanism is raised by frictional heat with heightening power, the gear is durable. Moreover, the thermosetting synthetic resin is filled with aramid fibers having a low attacking property to a counter member and an effect of improving the abrasion resistance of the synthetic resin, one or both of the driving gear and driven gear filled with the aramid fibers has an excellent abrasion resistance and toughness and further enables a reduction in the abrasion of the gear as the meshing counterpart, thereby improving the durability of both the gears. Furthermore, since the aramid fibers have a small thermal expansion coefficient and good heat stability, the gears can have excellent dimensional stability. In particular, since a para-linked aramid fiber has a negative thermal expansion coefficient, it is convenient to limit the thermal expansion of the thermosetting synthetic resin. However, since the para-linked aramid fiber is extremely tough, it is difficult to form the teeth of the gear by cutting. Therefore, it is preferred to mix the para-linked aramid fibers with meta-linked aramid fibers that have slightly lower toughness but have good workability and to fill the mixed fibers.
An electric power steering apparatus according to the second invention is characterized in that the aramid fibers are formed into a sheet. Here, the sheet is, for example, felt obtained by needling a pile of the aramid fibers to couple the respective fibers, woven material or knitted material made by using threads of the aramid fibers. The needling is a process of putting a needle having a hook into the fiber pile and drawing the needle out of the fiber pile in the direction of the thickness of the fiber pile so that the fibers caught by the hook in this action are oriented in the thickness direction. The fibers oriented in the thickness direction perform the function of coupling the fibers oriented in the plane direction.
According to the second invention, since the positional arrangement of the aramid fibers and the distance between the aramid fibers can be freely set, it is possible to further increase the abrasion resistance and toughness of the gear filled with the sheet.
An electric power steering apparatus according to the third invention is characterized in that the sheet is wound repeatedly into a cylindrical shape and further folded like bellows in an axial direction of the cylindrical shape.
According to the third invention, since the sheet of the aramid fibers are wound repeatedly into the cylindrical shape and folded in the axial direction of the cylindrical shape, the aramid fibers form circular fiber layers and are three-dimensionally positioned. Therefore, even when the teeth are formed by cutting, it is possible to further increase the abrasion resistance and toughness of the tooth portion.
When this structure is applied particularly to the worm wheel by selecting the above-mentioned felt as the sheet, the contact faces of the folds of the sheet are stacked in the axial direction, so that the shear force exerted from the worm is spread and received evenly by the respective contact faces. It is therefore possible to further improve the strength and durability.
On the other hand, when a structure where the sheet is simply wound repeatedly is adopted, the contact faces of the wound sheet are stacked in a radial direction, so that the direction of the shear force exerted from the worm and the direction of the contact faces agree with each other. Therefore, a separation phenomenon tends to occur at the contact faces, and thus it is not preferred to adopt such a structure.
An electric power steering apparatus according to the fourth invention is characterized in that the sheets are stacked in layers.
According to the fourth invention, since sheets of the aramid fibers are layered to form a plurality of fiber layers, it is possible to equalize the density of the aramid fibers, thereby further increasing the abrasion resistance and toughness of the tooth portion even when the teeth are formed by cutting.
An electric power steering apparatus according to the fifth invention is characterized in that the aramid fibers are filled in a ratio of 20 to 60% by volume.
According to the fifth invention, since the aramid fibers filled in a ratio of 20 to 60% by volume reinforce the thermosetting synthetic resin, one or both of the driving gear and driven gear have excellent abrasion resistance and toughness and improved durability.
Further, when the filling amount of the aramid fibers is less than 20% by volume, the brittleness as a characteristic of the thermosetting synthetic resin is increased, and consequently the thermosetting synthetic resin can not be satisfactorily reinforced. Thus, it is not preferred to fill the aramid fibers in an amount less than 20% by volume. On the other hand, when the filling amount of the aramid fibers exceeds 60% by volume, it is impossible to sufficiently fill the thermosetting synthetic resin into the respective fibers, resulting in insufficient reinforcement of the thermosetting synthetic resin by the aramid fibers. Therefore, it is not preferred to fill the aramid fibers in an amount more than 60% by volume.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.